Surface-enhanced infrared absorption (SEIRA) has been gaining substantial attention by using plasmonic nanoantennas to amplify near-field intensities so that it can extend IR spectroscopy to zeptomolar quantities and ultimately to the sigle-molecule level. Here we report a new nanoantenna for SEIRA detection, consisting of a fan-shaped Au structure positioned at a well-specified distance above a reflective plane with an intervening silica spacer layer. This antenna can be easily tuned to overlap vibrational modes within a broad spectral range from the near-IR into terahertz regimes. Our finite difference time domain (FDTD) simulations reveal a maximum SEIRA enhancement factor of 105 in the antenna junction area, which is corresponding to the experimental detection of 20-200 zeptomoles of octadecanethiol, using a standard commercial FTIR spectrometer. Our optimized antenna exhibits an order of magnitude greater SEIRA sensitivity than previous record-setting designs, which opens new opportunities for using infrared spectroscopy to analyze exceptionally small quantities of molecules.
The use of surface plasmons, charge density oscillations of conduction electrons of metallic nanostructures, could drastically alter how sunlight is converted into electricity or fuels by increasing the efficiency of light-harvesting devices through enhanced light-matter interactions. Surface plasmons can decay directly into energetic electron-hole pairs, or “hot” carriers, which can be used for photocurrent generation or photocatalysis. However, little has been understood about the fundamental mechanisms behind plasmonic carrier generation. Here we use metallic nano-wire based hot carrier devices on a wide-bandgap semiconductor substrate to show that plasmonic hot carrier generation is proportional to field intensity enhancement instead of bulk material absorption. We also show that interband carrier generation results in less energetic carriers than plasmon-induced generation, and a plasmon is required to inject electrons over a large energy barrier. Finite Difference Time Domain (FDTD) method is used for theoretical calculations, which match well with experimental results. This work points to a clear route to increasing the efficiency of plasmonic hot carrier devices and drastically simplifies the theoretical framework for understanding the mechanisms of hot carrier generation.